Galling – cold welding of mating threads – is one of the most common problems with stainless steel and nickel-alloy fasteners. A galled nut seizes on the stud, cannot be removed and often damages both parts. Many galling problems can be prevented by decisions made before the fasteners are bought.
Why Stainless Fasteners Gall
Austenitic stainless steels and many nickel alloys rely on a thin oxide film. Under thread pressure and sliding, that film breaks, clean metal surfaces touch and bond, and material tears from one surface onto the other. High tightening speed, dirty or damaged threads and same-material pairs make it worse.
Questions to Resolve Before Procurement
- Material pairing: can the nut be a different grade or condition from the bolt (for example a harder or different alloy nut), within the design code?
- Condition and hardness: strain-hardened (Class 2) or annealed (Class 1) bolting, and the hardness difference between bolt and nut.
- Thread finish and fit: rolled threads and an appropriate class of fit reduce galling risk.
- Lubricant or coating: an anti-seize compound or a thread coating suited to the service temperature and fluid.
- Installation: tightening method and speed, especially with power tools.
At Installation
- Keep threads clean and protect them during handling.
- Apply the specified anti-seize to the threads and bearing faces.
- Tighten at a steady, moderate speed; stop if the torque rises abnormally.
- Do not reuse nuts that show galling marks.
Related: fastener RFQ template and 304H vs 309 fasteners at high temperature.
Frequently Asked Questions
What causes galling in stainless steel fasteners?
Pressure and sliding between threads break the protective oxide film, so clean metal surfaces bond and tear. Same-material pairs, dirty threads and fast tightening make it more likely.
Does anti-seize prevent galling?
A suitable anti-seize compound reduces friction and is one of the most effective measures, provided it is compatible with the service temperature and process fluid.
Can I use a different material for the nut?
Often a different grade or condition for the nut reduces galling, but the combination must be allowed by your design code and specification.
